Because of its clean burning qualities and convenience, natural gas has become widely used in recent years. Many sources of natural gas are located in remote areas, great distances from any commercial markets for the gas. Sometimes a pipeline is available for transporting produced natural gas to a commercial market. When pipeline transportation is not feasible, produced natural gas is often processed into liquefied natural gas (which is called "LNG") for transport to market.
Natural gas often contains diluent gases such as nitrogen and helium. The presence of these gases reduces the heating value of the natural gas. Also, certain of these gases may have independent commercial uses if they can be separated from the natural gas. Consequently, the separation of diluent gases from natural gas may have twofold economic benefit, namely, enhancement of the natural gas heating value and production of a marketable gas such as helium. LNG plants also remove the nitrogen from the natural gas because the nitrogen will not remain in the liquid phase during transport of conventional LNG, which is at or near atmospheric pressure.
In general, most known natural gas separation processes comprise at least three distinct operative steps or stages. These include (1) a preliminary gas treatment step for the removal of water and acidic gases such as carbon dioxide and hydrogen sulfide, (2) a natural gas liquids product separation step using low but non-cryogenic temperatures for the separation and recovery of the ethane and heavier hydrocarbon components, and (3) a nitrogen separation or rejection step, often referred to as Nitrogen Rejection Units (NRUs). The nitrogen rejection is generally effected by the cooling of the nitrogen-containing natural gas and fractionating it in a distillation column.
It has recently been proposed to produce methane-rich liquid having a temperature above about -112.degree. C. (-170.degree. F.) and a pressure sufficient for the liquid to be at or below its bubble point. This pressurized liquid natural gas is sometimes referred to as PLNG to distinguish it from LNG which is at or near atmospheric pressure. The pressure of PLNG will typically be above about 1,380 kPa (200 psia). One of the advantages of a process for producing PLNG is that pressurized liquefied natural gas can contain up to about 10 mole percent nitrogen. However, the nitrogen lowers the heating value of the PLNG and increases the bubble point of the PLNG product. There is therefore a need for an improved process for removing nitrogen from a pressurized natural gas stream and simultaneously producing PLNG.